U.S. patent number 10,050,549 [Application Number 15/421,766] was granted by the patent office on 2018-08-14 for power converter unit including a rectifier and an active power filter.
This patent grant is currently assigned to GOODRICH CONTROL SYSTEMS. The grantee listed for this patent is Goodrich Control Systems. Invention is credited to Thomas Gietzold, Francisco Gonzalez-Espin.
United States Patent |
10,050,549 |
Gonzalez-Espin , et
al. |
August 14, 2018 |
Power converter unit including a rectifier and an active power
filter
Abstract
A power converter unit comprising a rectifier arranged to
receive AC input from a variable or fixed frequency AC power source
and an active power filter with an adaptive control algorithm
connected as a shunt between the AC input and the rectifier.
Inventors: |
Gonzalez-Espin; Francisco
(Madrid, ES), Gietzold; Thomas (Stratford upon Avon,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Goodrich Control Systems |
West Midlands |
N/A |
GB |
|
|
Assignee: |
GOODRICH CONTROL SYSTEMS
(Solihull, West Midlands, GB)
|
Family
ID: |
55272395 |
Appl.
No.: |
15/421,766 |
Filed: |
February 1, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170222571 A1 |
Aug 3, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Feb 1, 2016 [EP] |
|
|
16153595 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M
7/06 (20130101); H02M 1/08 (20130101); H02M
1/12 (20130101); H02M 1/15 (20130101); H02J
3/01 (20130101); H02M 2001/0009 (20130101); Y02T
50/50 (20130101); Y02E 40/20 (20130101) |
Current International
Class: |
H02M
7/00 (20060101); H02M 7/06 (20060101); H02M
1/12 (20060101); H02M 1/08 (20060101); H02M
1/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
An adaptive synchronous-Reference-Frame Phase-Locked-Loop for Power
Quality Improvement in a Polluted Utility Grid. F. Gonzalez-Espin,
E. Figueres, and G. Garcera, IEEE Transactions on Industrial
Electronics, vol. 59, pp. 2718-2731, 2012. cited by applicant .
Extended European Search Report, partial European Search
Report/Declaration of No Search, and European Search Opinion of
European Patent Application No. 16153595.0, dated Jul. 13, 2016, 7
pages. cited by applicant .
Hogan Diarmaid J. et al., "Adaptive Resonant Current-Control for
Active Power Filtering within a Microgrid", 2014 IEEE Energy
Conversion Congress and Exposition (ECCE), IEEE, Sep. 14, 2014, pp.
3468-3475, (retrieved on Nov. 11, 2014). cited by applicant .
Multipulse AC-DC Converters for Improving Power Quality: A Review,
B. Singh, S. Gairola, B.N. Singh, A. Chandra; K. Al-Haddad. IEEE
Transactions on Power Electronics, vol. 23, No. 1, Jan. 2008, pp.
260 ff. cited by applicant .
Polyphase Transformer Arrangements with Reduced kVA Capacities for
Harmonic Current Reduction in Rectifier-Type Utility Interface, S.
Choi, P.N. Enjeti and I.J. Pitel. IEEE Transactions on Power
Electronics, vol. 11, pp. 680-690, 1996. cited by
applicant.
|
Primary Examiner: Berhane; Adolf
Assistant Examiner: Lee, III; Henry
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
The invention claimed is:
1. A power converter unit comprising: a rectifier arranged to
receive AC input from an AC power source; an active power filter
connected as a shunt between the AC input and the rectifier,
wherein the active power filter comprises a control algorithm to
cause removal of current harmonic frequencies from the AC input but
not of the current fundamental frequency of the AC input; means for
transforming sensed AC input currents to a synchronous reference
frame; means for filtering out a fundamental harmonic from a sensed
load current and providing only harmonics of the fundamental at an
output; adaptive current control means for providing a high control
loop gain at the frequency of the harmonics from the output of the
means for filtering regardless of AC power source fundamental
frequency; and means for modulating the output of the adaptive
current control means.
2. The power converter unit of claim 1 where the rectifier is
arranged to receive AC input from a fixed or variable frequency AC
power source.
3. The power converter unit of claim 1, wherein the rectifier
comprises a 3-phase diode bridge.
4. The power converter unit of claim 1, wherein the active power
filter comprises a plurality of semiconductor switches and a
storage element.
5. The power converter unit of claim 1, wherein the adaptive
current control includes adaptive band-pass filters implemented
using a Schur-Lattice LLR structure.
6. The power converter unit of claim 1, wherein the control
algorithm further comprises means for synchronising the active
power filter with the AC power source.
7. The power converter unit of claim 6, wherein the means for
synchronising is an algorithm using Adaptive Lattice Synchronous
Reference Frame Phase Locked Loop (ALSRF-PLL) synchronisation.
8. The power converter unit of claim 7, wherein the algorithm
comprises means for transforming an AC voltage vector of the AC
input to a synchronous reference phase; a regulator and an
integrator connected in series to provide an input to the means for
transforming; a plurality of adaptive band-stop filters arranged to
reject undesired variable frequency harmonic content from the
sensed AC input voltage, regardless of AC power source fundamental
frequency.
9. The power converter unit of claim 8, wherein the band-stop
filters are implemented using a Schur-Lattice IIR structure.
10. The power converter unit of claim 9, wherein the band-stop
filters centre frequencies are adapted in real-time to track AC
power source frequency variations using a Gradient Lattice
Algorithm.
Description
FOREIGN PRIORITY
This application claims priority to European Patent Application No.
16153595.0 filed Feb. 1, 2016, the entire contents of which is
incorporated herein by reference.
TECHNICAL FIELD
The present disclosure is related generally to aircraft electrical
power systems and more specifically to a rectifier unit for an AC
to DC power converter.
BACKGROUND
In aircraft, there is now a move to greater use of electrical power
and electronic systems rather than hydraulic and pneumatic systems
as this has the potential of leading to lighter aircraft. This
trend to more electric aircraft (MEA) means that modern aircraft
include large generators that generate more electrical power during
flight than today and this generated power is used to supply power
to more and different on-board aircraft electric power systems. The
generators use rotation of the engine to generate AC power using
known techniques. The electric frequency on more recent aircrafts
may range from 350-800 Hz, while the AC voltage is usually
regulated at a fixed value such as 115 Vac or 230 Vac. While the
aircraft engines are not running, the on-board electric systems are
generally powered by ac power from a ground cart. Such power is
typically 115V/230V 400 Hz AC power.
While aircraft electrical power is generated in the form of
three-phase alternating current, most electrical loads require DC
power to operate, and so conversion of AC to DC power is required.
The direct rectification of 3-phase AC power into DC power is
simple and straightforward, e.g. using 3 pairs of diodes, however
this creates unacceptable levels of current distortion or
harmonics. These distortions cause power quality issues which are
difficult to address, especially since, in modern aircraft, the
fundamental frequency may vary over a wide range. Regulating bodies
have imposed stringent power quality requirements including
limitations on harmonic currents that can be drawn from an aircraft
ac power system.
Multipulse power conversion is one of several technologies capable
of AC-DC power conversion with low distortion levels that meet
aerospace power quality standards.
A typical autotransformer-based multipulse converter contains two
major functional blocks--a multipulse autotransformer and a
rectifier. Autotransformer rectifier units (ATRU) have a low part
count and are highly reliable. They have only few low frequency
switching components, so EMI emissions are low. However the ATRU
adds considerable weight and cost to an aircraft system equipped
that way.
A multiphase converter performs a phase shifting process through
transformers to convert from an original three-phase ac supply to
multiphase ac supply to result in a higher number of pulses in dc
output to result in a close to sinusoidal current draw with reduced
harmonic distortion at the ATRU input.
An n-pulse ATRU is composed of n/6 6-pulse diode bridges (n/2 diode
pairs) and uses phase-shifting of the secondary voltages in the
autotransformer. A three phase fixed or variable
frequency--constant amplitude voltage source supplies power to the
ATRU, thus providing three different phase shifted sinusoidal 3
phase voltages
ATRUs used in aircraft are typically 18-pulse converters, but
others, e.g. 12-pulse, 24-pulse, etc. may be used. A 12-pulse ATRU,
for example, is described in "Polyphase Transformer Arrangements
with Reduced kVA Capacities for Harmonic Current Reduction in
Rectifier-Type Utility Interface", S. Choi, P. N. Enjeti and I. J.
Pitel. IEEE Transactions on Power Electronics, vol. 11, pp 680-690,
1996. An overview of multipulse AC-DC converters can be found in
"Multipulse AC-DC Converters for Improving Power Quality: A
Review", B. Singh, S. Gairola, B. N. Singh, A. Chandra; K.
Al-Haddad. IEEE Transactions on Power Electronics, vol. 23, No. 1,
January 2008, pp. 260 ff.
FIG. 1 shows a block diagram of a typical 18-pulse rectifier unit
including the Autotransformer Rectifier Unit (ATRU), diode bridges
and Interphase Transformers (IPTs). This shows the ATRU 1
comprising an isolated or non-isolated multi-winding transformer
that displaces the AC input voltage a number of degrees dependent
on the number of secondary windings.
The output voltage of the multi-phase winding is rectified by a
number of basic 6-pulse rectifier cells 2, thus improving the input
current total harmonic distortion. The input current quality,
however, is limited by the number of 6-pulse stages. If additional
6-pulse cells are added to achieve a lower distortion, this results
in an undesirable increase in the weight of the rectifier unit.
The aim of the present disclosure is to provide a power converter
that mitigates harmonic distortion and adds redundancy to the
rectification stage with either fixed frequency or variable
frequency AC power source, while avoiding the use of the
autotransformer unit and the IPT, thus reducing the rectifier unit
weight compared with the existing ATRU solution.
SUMMARY
According to the disclosure there is provided an AC-DC converter
unit comprising a rectifier to receive AC input from an AC power
source and an active power filter (APF) connected as a shunt
between the AC input and the rectifier.
The AC power source can be fixed or variable frequency.
The rectifier is preferably a single, 6-pulse rectifier cell.
The rectifier provides a DC output. The converter unit may further
comprise a filter connected to the rectifier DC output to smooth
the DC output.
The APF is controlled by an adaptive algorithm to reduce or
eliminate fixed or variable frequency harmonics drawn by the
rectifier, thus avoiding the need for an ATRU.
The rectifier unit can be operated in a degraded mode if the APF
does not perform as expected, thus adding redundancy to the
rectifier unit.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments will now be described, by way of example
only, with reference to the drawings.
FIG. 1 is a block diagram of a typical multipulse rectifier system
using an ATRU to reduce harmonic distortions;
FIG. 2 is a block diagram of a rectifier unit according to this
disclosure;
FIG. 3 is a block diagram showing the APF in more detail;
FIG. 4 shows an adaptive lattice SRF-PLL;
FIG. 5 shows an adaptive band-stop lattice filter;
FIG. 6 shows Schur recursion;
FIG. 7 shows the frequency response of the Schur Lattice notch
filter;
FIG. 8 shows an adaptive shunt APF current controller;
FIG. 9 shows an adaptive band-pass lattice filter;
FIG. 10 shows the frequency response of the Schur Lattice band-pass
filter.
DETAILED DESCRIPTION
The conventional rectifier unit has been described above in the
Background section with reference to FIG. 1.
In contrast, in the present disclosure, the ATRU and the use of n/6
6-pulse diode bridges (where n is an integer) is substituted by a
single 6-pulse rectifier cell connected directly to the AC power
distribution system (which can be fixed or variable frequency) and
an active power filter (APF) connected in parallel to the rectifier
cell, as a shunt APF.
The shunt APF is designed to locally compensate for the non-linear
current harmonics, thus allowing the harmonics content of the
current drawn by the power distribution system to be below the
permitted maximum.
The shunt APF control includes an adaptive control algorithm able
to assure proper current harmonics compensation within a predefined
frequency variation range, typically 350-800 Hz.
The active filter is arranged to modify the amplitude and/or phase
characteristics of the current provided by the AC power signal so
as to compensate for harmonic currents drawn by the load.
The proposed power converter unit is shown in FIG. 2, where block A
and block B represent the main parts of the unit. Both block A and
block B are connected to the AC power distribution system. Block A
is a rectifier composed of a 3-phase diode bridge 3, a filter 4 and
the load. The current drawn by the block A contains harmonics from
the fundamental AC power distribution frequency that need to be
filtered out to meet power quality requirements. As discussed
above, the current solution is to use an ATRU to eliminate or
minimise harmonics. The present disclosure presents an alternative
solution. In order to achieve the aforementioned power quality
requirements, a shunt active power filter or APF depicted in block
B is proposed in this disclosure. The APF is composed by at least
six semiconductors 5 operated in switching mode and a storage
element 6, which could be a capacitor. The APF is controlled so
that it locally injects the harmonics other than the fundamental
frequency drawn by the block A, so that a cancelling-out effect
occurs and the current provided by the AC power distribution
contains only the fundamental frequency. The APF shown in block B
includes an adaptive control algorithm which is able to compensate
the undesired harmonics drawn by block A no matter what the
variation in the frequency of the AC power distribution main
harmonic is, which typically varies from 350-800 Hz in a variable
frequency AC power supply system.
The preferred shunt active power filter is shown in more detail in
FIG. 3. Block C represents the power stage comprising the
semiconductors 5 and storage element 6 explained in the previous
section, while block D represents the control architecture of the
control algorithm to achieve the harmonics compensation regardless
of frequency variations of the AC power distribution system.
The preferred control algorithm is composed by: A transformation
stage, e.sup.-j{circumflex over (.theta.)}, 7 which rotates the
sensed currents from the natural reference frame to the synchronous
reference frame. A high pass filter, HP, 8 which rejects the
fundamental harmonic information from the sensed load current. The
output of the HP block is used as the reference for the current to
be injected by the shunt APF. An Adaptive current controller 9,
which is responsible for offering a high control loop gain at the
frequency of the harmonics to be injected by the shunt APF,
regardless the AC power system main frequency A Modulation block
10, which is typically deployed through pulse width modulation PWM
or space vector modulation SVM modulation techniques, and that may
include a transformation stage as well, An Adaptive grid
synchronization algorithm 11 which extracts AC power distribution
voltage frequency and phase angle information.
The adaptive features of block D are provided by the adaptive grid
synchronization algorithm 11 and the adaptive current controller 9
as described further below.
Adaptive Grid Synchronization Algorithm
The adaptive grid synchronization algorithm 11 is responsible for
synchronising the shunt APF with the AC power distribution system.
This algorithm is based on the standard Synchronous Reference Frame
Phase Locked Loop (SRF-PLL) and adds adaptive filtering features to
cope with the lack of synchronization accuracy that occurs when the
AC power distribution voltage is polluted with voltage harmonics
and its frequency is not fixed.
The preferred adaptive grid synchronization algorithm proposed in
this disclosure is called Adaptive Lattice Synchronous Reference
Frame Phase Locked Loop (ALSRF-PLL) and is shown in FIG. 4. The
ALSRF-PLL has been described in "An adaptive
synchronous-Reference-Frame Phase-Locked-Loop for Power Quality
Improvement in a Polluted Utility Grid." F. Gonzalez-Espin, E.
Figueres, and G. Garcera, IEEE Transactions on Industrial
Electronics, vol. 59, pp. 2718-2731, 2012, and is composed of three
main blocks: Block E represents the Park transformation, and is
responsible for rotating the AC voltage vector from the natural
reference frame to the synchronous reference frame. Block F
includes the regulator H.sub.PI(z), which is usually a
Proportional+Integral controller, and the block Int(z), which is an
integrator. The output of those blocks are the AC voltage
frequency, .omega., and phase angle, {circumflex over (.theta.)},
respectively. Block G adds the adaptive feature to the ALSRF-PLL
and is composed of n adaptive narrow band-stop (notch) filters,
G.sub.hn(z), that reject the undesired variable frequency harmonic
content from the sensed AC voltage, thus assuring an accurate
estimate of voltage frequency and phase angle.
The adaptive notch filters 12 are preferably implemented by using
the Schur-lattice IIR structure shown in FIG. 5, which carries out
the rotation over the transfer function involving the filtering
process shown in FIG. 6. The transfer function of the notch filter
is shown in equation (1), and can be derived from Block H, where
the normalized center frequency, .omega..sub.0, and the bandwidth,
BW, can be adjusted by equation (2) and (3), respectively. The Bode
plot of G(z) is depicted in FIG. 7 for .theta..sub.1=0 rad and
.theta..sub.2=1.414 rad.
.function..function..theta..times..times..times..function..theta..times..-
function..theta..times..function..theta..times..function..theta..times..om-
ega..theta..pi..times..theta.<.pi..times..function..function..theta..fu-
nction..theta. ##EQU00001##
The main advantages of using the Schur-lattice IIR structure are as
follows: i. The structure is inherently limited to realize stable
and causal filters. This makes it very appealing to use adaptive
algorithms to adjust its center frequency in real time. ii. As will
be explained below, a band-stop as well as a band-pass filter can
be obtained by using the same topology. iii. All the internal nodes
are inherently scaled in the Euclidean norm. In this regard, the
precision can be kept constant during the filtering process. iv.
The mapping of the poles and zeros is more precise regardless of
the position of the poles and zeros, because the round-off noise
accumulation in the state vector loop is inherently low. v.
Quantization limit cycles can be easily suppressed.
The Gradient Adaptive Lattice (GAL) recursive algorithm can be used
to automatically adjust in real-time the parameter .theta..sub.1,
so that the center frequency of the filters are able to filter out
the undesired variable frequency harmonics. An important feature of
the Schur Lattice filter with GAL recursive algorithm is that the
filter does not need a reference to adaptively tune its center
frequency, which is of great importance when filtering out the
ALSRF-PLL harmonics. Block I in FIG. 5 implements the GAL
algorithms expressed in equation (4), where .mu. is the adaptation
gain. .theta..sub.1(n+1)=.theta..sub.1(n)-.mu.y(n)x.sub.1(n-1)
(4)
Table I shows the implementation of the Schur Lattice IIR Notch
Filter with GAL adaptation algorithm.
TABLE-US-00001 TABLE I GAL ALGORITHM APPLIED TO THE SCHUR-LATTICE
IIR NOTCH FILTER Filter Parameters Computing
.function..function..function..theta..function..theta..function..theta..fu-
nction..theta..function..function..function. ##EQU00002##
.function..function..function..theta..function..function..theta..function.-
.function..theta..function..function..theta..function..function..function.-
.function. ##EQU00003## .function..function..function..function.
##EQU00004## Filter Parameters Adaptation .theta..sub.1(n + 1) =
.theta..sub.1(n) - .mu.y(n)x.sub.1(n - 1)
Adaptive Current Controller
The adaptive current controller is preferably based on the control
architecture shown in FIG. 8. The Proportional+Lattice controller
is composed of a gain, K.sub.D, in parallel with harmonic
compensators, H.sub.hn(z). The harmonic compensators are band-pass
filters tuned at the corresponding harmonics (hl to hn in the
figure). The band-pass filters offer a high gain in the control
loop, so the shunt APF is able to accurately compensate for the
harmonics drawn by the non-linear load. The information about the
frequency of the AC voltage estimated by the Adaptive grid
synchronization algorithm 11 (ALSRF-PLL) is used to adjust in real
time the band-pass filter center frequency, so that the tracking of
the harmonics is accurate through all of the frequency range of
operation, typically 350-800 Hz.
Schur Lattice IIR band-pass Filter structure shown in FIG. 9 is
preferably used in this disclosure and allows implementing an
inherently stable IIR filter, which is of paramount importance when
adjusting coefficients in real-time, so that the stability of the
closed loop APF current control is not compromised by the
coefficient adaptation process.
The transfer function of the Proportional+Lattice controller is
depicted in equation (5), where (2) and (3) applies.
.function..times..function..theta..times..times..function..theta..times..-
times..function..theta..times..times..function..theta..times..times.
##EQU00005##
The Bode plot of H.sub.PL(z) is depicted in FIG. 10 for .theta.1=0
rad, .theta..sub.2=1.414 rad and K.sub.L=1
The system of this disclosure, by doing away with the ATRU, reduces
the weight of the rectifier unit. Further, the shunt APF is
digitally controlled and can use high frequency pulse with
modulation (PWM) or other switching modulation techniques and an
adaptive control algorithm, which allows very accurate fixed or
variable frequency harmonics compensation. This significantly
decreases weight and size of the rectifier unit compared to known
multipulse rectifiers while obtaining comparable or better power
quality of the current injected by the electric power system,
regardless of frequency variations. The provision of the shunt APF
in parallel with the rectifier also provides redundancy--i.e. if
the APF fails, the rectifier will continue to operate, albeit in a
degraded mode, so that the load will still receive power even
though the current provided by the AC power system will be of a
lower quality with more harmonic distortion.
* * * * *